This invention relates to a method of recovering crude petroleum from subsurface earth formations. More particularly, this invention relates to a method for improving a steam foam diversion process to increase the steam sweep efficiency during steam-flooding for the recovery of oil from an underground formation.
Many hydrocarbons are too thick to be recovered from subterranean petroleum-containing formations without assistance. These hydrocarbons are either the residual oil left in a depleted reservoir or virgin heavy hydrocarbons. These heavy hydrocarbons can be recovered through the use of steam drives which heat the formation, lower the viscosity of the hydrocarbons and enhance the flow of the hydrocarbons toward a production well.
However, it is commonly found that the steam will find shortcut pathways to some of the producing wells, thus bypassing oil in the zone between the injection well and the production well. Also, after initial steam injection breakthrough at the production well, the steam injection preferentially follows the path of the breakthrough. These pathways can take the form of channels in the formation or of gravity override in the upper portion of the oil-bearing stratum. Gravity override results from the lower density and viscosity of steam vapor compared to liquid oil and water. Thus, the total amount of the formation that is swept by the steam injection is limited.
Various methods have been proposed to mitigate the loss of steam flow and heating value within the formation. For example, a number of commercial surfactants have been injected along with steam to create a steam-foam flood. The surfactants form a foam that inhibits the flow of the steam into that portion of the formation containing only residual oil saturation and serves to physically block the volumes through which steam is short-cutting. This forces the steam to drive the recoverable hydrocarbons from the less depleted portions of the reservoir toward the production well.
In addition, various inert or noncondensable gases have been added to the steam, both in the presence and absence of foaming surfactants, in order to enhance and maintain an oil-driving force within the formation.
For example, in U.S. Pat. No. 3,908,762, a complex steam injection process is described which employs a mixture of steam and a noncondensable gas. In this patent, the improvement is primarily based upon the disclosure that the noncondensable gas may include nitrogen, air, carbon dioxide, flue gas, exhaust gas, methane, natural gas and ethane.
U.S. Pat. No. 4,086,964 to Dilgren et al. describes the use of a foam-forming mixture of steam, noncondensable gas and surfactant injected into a steam channel in an oil reservoir in which stratification of the rock permeability is insufficient to confine steam within the permeable strata. The noncondensable gas which is added to the foam and steam is in very low concentration to stabilize the foam. The gas is included in fractions of a mole percent in the foam and the foam is intended to resist the flow of steam through the oil-depleted zone, thereby diverting the steam into undepleted zones.
U.S. Pat. No. 4,445,573 to McCaleb discloses a method for recovering oil from a subterranean oil-bearing reservoir by steam stimulation, wherein the reservoir includes a permeable zone and an oil-bearing strata, utilizing a mixture of noncondensable gas and surfactant which forms a relatively stable, substantially noncondensable, thermally insulating foam having the noncondensable gas as a gas phase. This noncondensable, thermally insulating foam is injected into the reservoir to substantially fill an expanse of the permeable zone in proximity with the portion of the oil-bearing strata to be stimulated.
U.S. Pat. No. 4,393,937 to Dilgren et al. discloses a steam foam drive process for displacing oil within a subterranean reservoir which utilizes a steam-foam-forming mixture of steam having a quality of 10 to 90 percent, and preferably 30 to 80 percent, an olefin sulfonate surfactant, an electrolyte and a noncondensable gas present in an amount between about 0.0003 and 0.3 mole percent of the gas phase of the mixture.
U.S. Pat. No. 4,161,217 to Dilgren et al. discloses a process for recovering oil from a subterranean reservoir by establishing a channel of preferential permeability through the reservoir between injection and production locations, then flowing through the reservoir a hot foam of aqueous liquid, noncondensable gas and surfactant, and controlling the mobility of the foam so that heated oil is produced and hot fluid is flowed through portions of the reservoir within and outside the channel of preferential permeability.
U.S. Pat. No. 4,085,800 to Engle et al. discloses a process for plugging a permeable strata in subterranean oil-bearing formations wherein a hot, noncondensable gas is used to preheat a portion of the permeable strata to a temperature above the boiling point of water at operating pressure. A mixture of steam and a surfactant is then introduced into the preheated portion of the permeable strata and allowed to form a foam, thereby plugging the permeable strata. Once the desired portion of the permeable strata is plugged, steam alone, without a surfactant, is utilized for stimulating production of oil from less permeable strata.
Demonstration of the ability of a surfactant to form a useful foam at steamflood conditions is commonly done by a laboratory coreflood. In this procedure, a cylindrical core of sandstone of relatively high permeability is encased in a pressure tight holder and fitted with means for passing gases and liquids through it. The usual instruments to record flow rates, temperatures, and pressure drop across the core allow measurements of resistance to flow to be made under simulated oil field reservoir conditions. A standard measure of the suitability of a surfactant to divert steam flow is the "Resistance Factor" (RF). The Resistance Factor is defined as the ratio of resistance to flow using steam and a surfactant compared to steam alone, i.e., the ratio of the pressure drops across the core under the two conditions. The higher the value of RF, the more effective the surfactant under a given set of conditions.
In the Society of Petroleum Engineers paper No. SPE 16375 (April, 1987) entitled "Physical and Chemical Effects of an Oil Phase on the Propagation of Foam in Porous Media", J. A. Jensen and F. Friedmann teach that RF values in laboratory cores decrease significantly when steam quality is decreased. The data in this paper also show that RF values increase with the injection of nitrogen.
Society of Petroleum Engineers paper No. SPE 13609 (March, 1985) entitled "Two Successful Steam-Foam Field Tests, Sections 15A and 26C, Midway-Sunset Field" by J. F. Ploeg and J. H. Duerksen and paper No. SPE 16736 (Sept., 1987) entitled "Steam-Foam Pilot Project in Dome Tumbador, Midway-Sunset Field" by S. S. Mohammadi et al. disclose oil field steam-foam applications using 24 and 18 standard cubic feet of nitrogen gas per cold water equivalent barrel of steam injected, respectively.
In view of the foregoing, it can be seen that it is known in the art to combine the heating of a reservoir with steam to increase the mobility of crude oil therein with the injection of foamable surfactant and small amounts of noncondensable gas to improve the sweep efficiency of the steam within the reservoir. There is a need, however, to provide a practical means to accomplish these goals more efficiently.